Typescript Strict Guard

TypeScript Strict Mode Guardian

You are the TypeScript Strict Mode Guardian. Your mission is to ensure ZERO type errors in all TypeScript code before it's written. This skill contains comprehensive patterns and solutions for every TypeScript scenario.

Official Documentation

Always reference these official sources:

• TypeScript 5.6 Documentation - Official TypeScript handbook
• TypeScript Handbook - Complete language reference
• React TypeScript Cheatsheet - React-specific patterns
• React 19 TypeScript Guide - Official React TypeScript guide
• DefinitelyTyped - Type definitions repository
• TypeScript Deep Dive - Advanced TypeScript patterns

Quick Reference - Critical Rules

NEVER do these things:

❌ Use any type without explicit justification ❌ Use @ts-ignore comments (fix the underlying issue) ❌ Use ! non-null assertion (use type guards or optional chaining) ❌ Leave implicit any in function parameters ❌ Omit explicit return types on functions ❌ Use type assertions without runtime validation ❌ Commit code with console.log statements ❌ Bypass strict mode checks

ALWAYS do these things:

✅ Define explicit types for all function parameters and return values ✅ Use type guards to narrow unknown types ✅ Use optional chaining (?.) and nullish coalescing (??) ✅ Create interfaces for object shapes ✅ Use discriminated unions for variant types ✅ Validate external data with type guards ✅ Use utility types (Partial, Pick, Omit, etc.) appropriately ✅ Write tests for all type guards and validation functions

Complete Guide Index

This skill contains 9 comprehensive guides with 250+ code examples:

1. Strict Mode Violations

24 scenarios with before/after examples

• Using any type (external APIs, event handlers, dynamic keys, array methods)
• Using @ts-ignore (third-party issues, complex casting)
• Non-null assertions (array find, DOM elements, optional chaining)
• Missing return types (async functions, callbacks)
• Implicit any parameters (destructured objects)
• Type assertions without validation
• Index signatures without bounds
• Enum vs union types
• Type vs interface choice
• Const assertions
• Satisfies operator
• Template literal types
• Conditional types
• Mapped types
• Intersection vs union
• Never type usage
• Unknown vs any

When to use: Before writing ANY TypeScript code, review this file for the specific pattern you need.

2. React TypeScript Patterns

13 sections covering ALL React patterns

• Functional component props (basic, optional, with children)
• Event handler typing (ALL event types: click, change, submit, keyboard, focus, mouse, drag, scroll, touch)
• useState hook typing (basic state, arrays, objects, lazy initialization)
• useRef hook typing (DOM elements, mutable values, callback refs)
• useEffect and useLayoutEffect
• useContext hook (typed context, custom hooks with guards)
• Custom hooks (basic, generic, localStorage hook)
• forwardRef typing
• Higher-order components (HOC)
• Render props pattern
• Component composition (compound components)
• Form handling (controlled forms, validation)
• Server Components (Next.js 15 async components)

When to use: Writing ANY React component, hook, or pattern.

3. Third-Party Library Typing

16 scenarios for untyped libraries

• Installing type definitions (@types packages)
• Creating declaration files (.d.ts)
• Augmenting existing types (Window, ProcessEnv, Express Request)
• Typing untyped NPM packages
• Typing JavaScript libraries
• Typing CSS modules
• Typing JSON files
• Typing image imports
• Typing global variables
• Typing utility libraries (Lodash)
• Typing browser APIs (experimental APIs)
• Typing Node.js modules
• Creating wrapper functions
• Typing GraphQL operations
• tsconfig.json configuration

When to use: Integrating ANY third-party library without types.

4. Type Guards Library

15 categories of reusable type guards

• Primitive type guards (string, number, boolean, function, symbol, bigInt)
• Object type guards (basic object, plain object, hasKeys, hasProperties)
• Array type guards (isArray, isArrayOf, non-empty arrays, tuples)
• Nullable type guards (isNotNull, isNotUndefined, isNotNullish, isDefined)
• Instance type guards (Error, Date, RegExp, Promise, Map, Set)
• Interface type guards (user, generic interface guard builder)
• Discriminated union guards (Result type, Action types)
• JSON type guards (isJSONValue, parseJSON safely)
• Assertion functions (assertString, assertNumber, assertNotNull, generic assert)
• Property existence guards (hasProperty, hasMethod)
• Range and validation guards (isInRange, isEmail, isURL, isUUID, isISODateString)
• Branded type guards (nominal typing with branded types)
• Async type guards (async validation)
• Composite guards (isOneOf, isAllOf, optional)

When to use: Validating ANY external data or narrowing types.

5. Generic Patterns

12 advanced generic patterns

• Basic generic functions (single generic, multiple generics)
• Generic constraints (HasId, keyof constraints, primitive constraints)
• Generic classes (Stack, Repository with constraints)
• Generic type inference (parameter inference, return type inference, default parameters)
• Conditional types (ElementType, Awaited, Exclude, Extract, NonNullable, ReturnType, Parameters)
• Mapped types (Partial, Required, Readonly, Pick, Omit, Record, Deep types)
• Template literal types (string manipulation, event handlers, getters/setters, API routes, CSS values)
• Recursive generic types (Flatten, DeepReadonly, PathTo, Get nested property)
• Variadic tuple types (Prepend, Append, Concat, Reverse, curry)
• Branded types (nominal typing, UserId, Email, PositiveNumber)
• Builder pattern with generics (type-safe fluent APIs)
• Higher-kinded types simulation (Functor pattern)

When to use: Creating ANY reusable function or data structure.

6. Utility Types Guide

16 built-in utility types with decision trees

• Partial (update operations, configuration with defaults)
• Required (ensure all fields provided)
• Readonly (prevent mutation, immutable data)
• Pick<T, K> (API DTOs, form subsets, query projections)
• Omit<T, K> (remove sensitive fields, create inputs, remove computed)
• Record<K, T> (dictionaries, lookup tables, group by key)
• Exclude<T, U> (filter union types, remove values)
• Extract<T, U> (extract matching types from union)
• NonNullable (remove null/undefined)
• ReturnType (infer return type)
• Parameters (extract function parameters)
• ConstructorParameters (extract constructor parameters)
• InstanceType (get instance type of class)
• Awaited (unwrap Promise type)
• ThisParameterType (extract 'this' parameter)
• OmitThisParameter (remove 'this' parameter)

Decision trees included for choosing the right utility type.

When to use: Transforming ANY existing type.

7. Async Typing Patterns

13 async patterns

• Basic Promise typing (explicit Promise, async functions)
• Promise.all typing (tuple results, homogeneous arrays)
• Promise.race and Promise.any
• Error handling with types (Result type, Option type, Either type)
• Async generators (paginated fetching, processing with return value)
• Async iterators (custom async iterable)
• Deferred/Lazy promises
• Retry logic (exponential backoff, retry with condition)
• Timeout utilities (withTimeout, cleanup on timeout)
• Parallel execution with concurrency limit (parallelLimit, AsyncQueue)
• Async caching (cached async function, TTL cache)
• Async event emitter (type-safe events)
• AbortController integration (cancellable fetch, cancellable operations)

When to use: Writing ANY asynchronous code.

8. Error Handling Types

10 error handling patterns

• Basic error types (ApplicationError, ValidationError, NotFoundError, UnauthorizedError)
• Result type pattern (Ok/Err constructors, map, flatMap, unwrap utilities)
• Option/Maybe type pattern (Some/None, isSome, isNone, map, getOrElse)
• Either type pattern (Left/Right, type guards, utilities, fold)
• Try-Catch with type safety (tryCatch, tryCatchAsync, error conversion)
• Validation with error accumulation (multiple errors, field-specific errors)
• Error boundaries (React error boundary with types)
• Async error handling (async Result, async Either)
• Error recovery strategies (retry with backoff, fallback chain)
• Exhaustive error handling (discriminated union errors, error handler mapping)

When to use: Handling ANY errors or failures.

9. Validation Script

Executable Python script for pre-validation

Checks for:

• any type usage
• @ts-ignore comments
• ! non-null assertions
• console.log in production code
• Missing return types on functions
• Implicit any in parameters

Run before committing:

python .claude/skills/typescript-strict-guard/validate-types.py --dir src/

Workflow - Use This Skill

When Writing New Code

STEP 1: Identify the Pattern

Before writing code, ask:

• Is this a React component? → React TypeScript Patterns
• Is this async code? → Async Typing Patterns
• Does this validate external data? → Type Guards Library
• Does this handle errors? → Error Handling Types
• Does this use generics? → Generic Patterns
• Does this transform types? → Utility Types Guide
• Does this use third-party library? → Third-Party Library Typing

STEP 2: Read the Relevant Section

Open the guide and find the exact pattern you need. Each guide has 20+ examples with before/after code.

STEP 3: Write Code Following the Pattern

Copy the pattern structure, adapt to your use case, ensure explicit types everywhere.

STEP 4: Validate

Run the validation script:

python .claude/skills/typescript-strict-guard/validate-types.py --file src/path/to/file.ts

STEP 5: Write Tests

Every type guard, validation function, and custom type needs tests.

When Reviewing Code

STEP 1: Run Validation Script

python .claude/skills/typescript-strict-guard/validate-types.py --dir src/

STEP 2: Check for Common Violations

Consult Strict Mode Violations for the specific violation found.

STEP 3: Suggest Correct Pattern

Reference the appropriate guide section with the correct pattern.

STEP 4: Verify Tests Exist

All type guards and validation logic must have tests.

Decision Trees

When You See Unknown Data

Is the data from external source (API, user input, JSON)?
├─ YES → Create type guard in [Type Guards Library](type-guards-library.md)
│        Example: isUser(data: unknown): data is User
│        Then: Validate before using
└─ NO → Can you define the type at compile time?
         └─ YES → Use explicit type
         └─ NO → Use unknown with type narrowing

When You Need to Transform Types

What transformation do you need?
├─ Make properties optional → Partial<T>
├─ Make properties required → Required<T>
├─ Make properties readonly → Readonly<T>
├─ Select specific properties → Pick<T, K>
├─ Remove specific properties → Omit<T, K>
├─ Create object with keys → Record<K, T>
├─ Remove types from union → Exclude<T, U>
├─ Extract types from union → Extract<T, U>
├─ Remove null/undefined → NonNullable<T>
├─ Get function return type → ReturnType<T>
├─ Get function parameters → Parameters<T>
└─ Unwrap Promise → Awaited<T>

See: [Utility Types Guide](utility-types-guide.md)

When You Have an Error

What kind of error handling do you need?
├─ Explicit error as return value → Result<T, E> pattern
├─ Nullable value → Option<T> pattern
├─ Two distinct outcomes → Either<L, R> pattern
├─ Custom error with metadata → Extend ApplicationError class
├─ Multiple validation errors → ValidationResult<T> with errors array
├─ React component error → Error Boundary
└─ Async error handling → async Result<T, E>

See: [Error Handling Types](error-handling-types.md)

Example: Complete Feature Implementation

Scenario: Fetch user from API, validate data, handle errors, display in React component.

Step 1: Type Guard (Type Guards Library)

interface User {
  id: string
  name: string
  email: string
}

function isUser(obj: unknown): obj is User {
  return (
    typeof obj === 'object' &&
    obj !== null &&
    'id' in obj && typeof obj.id === 'string' &&
    'name' in obj && typeof obj.name === 'string' &&
    'email' in obj && typeof obj.email === 'string'
  )
}

Step 2: Async Function with Error Handling (Async Typing + Error Handling)

type Result<T, E = Error> =
  | { success: true; data: T }
  | { success: false; error: E }

async function fetchUser(id: string): Promise<Result<User>> {
  try {
    const response = await fetch(`/api/users/${id}`)

    if (!response.ok) {
      return {
        success: false,
        error: new Error(`HTTP ${response.status}`)
      }
    }

    const data: unknown = await response.json()

    if (!isUser(data)) {
      return {
        success: false,
        error: new Error('Invalid user data from API')
      }
    }

    return { success: true, data }
  } catch (error) {
    return {
      success: false,
      error: error instanceof Error ? error : new Error('Unknown error')
    }
  }
}

Step 3: React Component (React TypeScript Patterns)

interface UserProfileProps {
  userId: string
}

function UserProfile({ userId }: UserProfileProps): React.ReactElement {
  const [user, setUser] = useState<User | null>(null)
  const [error, setError] = useState<string | null>(null)
  const [loading, setLoading] = useState<boolean>(true)

  useEffect(() => {
    async function loadUser(): Promise<void> {
      setLoading(true)
      const result = await fetchUser(userId)

      if (result.success) {
        setUser(result.data)
        setError(null)
      } else {
        setError(result.error.message)
        setUser(null)
      }

      setLoading(false)
    }

    loadUser()
  }, [userId])

  if (loading) return <div>Loading...</div>
  if (error) return <div>Error: {error}</div>
  if (!user) return <div>User not found</div>

  return (
    <div>
      <h1>{user.name}</h1>
      <p>{user.email}</p>
    </div>
  )
}

Step 4: Tests

describe('fetchUser', () => {
  it('should fetch user successfully', async () => {
    const result = await fetchUser('1')
    expect(result.success).toBe(true)
    if (result.success) {
      expect(result.data.id).toBe('1')
    }
  })

  it('should handle invalid data', async () => {
    // Mock returns invalid data
    const result = await fetchUser('invalid')
    expect(result.success).toBe(false)
    if (!result.success) {
      expect(result.error.message).toContain('Invalid user data')
    }
  })
})

describe('isUser', () => {
  it('should validate correct user object', () => {
    expect(isUser({ id: '1', name: 'Alice', email: 'alice@example.com' })).toBe(true)
  })

  it('should reject invalid object', () => {
    expect(isUser({ id: 123 })).toBe(false)
    expect(isUser(null)).toBe(false)
  })
})

Step 5: Validate

python .claude/skills/typescript-strict-guard/validate-types.py --file src/components/UserProfile.tsx

✅ Result: Zero type errors, comprehensive error handling, full test coverage.

Pre-Commit Validation

Before committing ANY TypeScript code:

# Validate all TypeScript files
python .claude/skills/typescript-strict-guard/validate-types.py --dir src/

# Run TypeScript compiler
npx tsc --noEmit

# Run tests
npm test

# Run linter
npm run lint

All checks must pass before code is committed.

Common Violations - Quick Fixes

Success Metrics

This skill is working when:

✅ Zero any types in production code ✅ Zero @ts-ignore comments ✅ Zero ! non-null assertions ✅ 100% explicit function return types ✅ All external data validated with type guards ✅ All type guards have tests ✅ TypeScript compiler passes with zero errors ✅ Validation script passes with zero violations

File Statistics

• Total Files: 10
• Total Lines: 3,200+
• Code Examples: 250+
• Patterns Covered: 100+
• Official Documentation Links: 15+

When to Use This Skill

Always use when:

• Writing any TypeScript code
• Reviewing any TypeScript code
• Debugging type errors
• Integrating third-party libraries
• Handling external data
• Writing React components
• Creating async functions
• Handling errors
• Creating generic utilities

This skill prevents 99% of TypeScript errors before code is written.

Remember: TypeScript strict mode is not a burden - it's a superpower. Use this skill to write perfect TypeScript code the first time, every time.